Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
  • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
  • G02B6/122—Basic optical elements, e.g. light-guiding paths
  • G02B6/1228—Tapered waveguides, e.g. integrated spot-size transformers
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/30—Optical coupling means for use between fibre and thin-film device
  • G02B6/305—Optical coupling means for use between fibre and thin-film device and having an integrated mode-size expanding section, e.g. tapered waveguide
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
  • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
  • G02B2006/12083—Constructional arrangements
  • G02B2006/12097—Ridge, rib or the like

Definitions

• the present invention generally relates to planar 5 integrated optical waveguides, and, more particularly, to integrated optical waveguides having medium to high refractive index contrast values .
• Low refractive index contrast integrated optical waveguides i.e., waveguides characterized by a
• ID refractive index contrast of less than approximately 1%) have traditionally been used in integrated optical devices because, having relatively wide cross-sections, the dimensions of the optical modes supported by these waveguides are comparable to those of standard optical
• ⁇ D from one optical guiding structure to the other strongly depends on how well the optical modes supported by each of the two optical guiding structures overlap.
• the overlap integral between the modes supported by the two guiding structures is usually taken as a measure of the
• 3D of integrated optical devices has increased.
• High integration scales have been achieved using medium to high refractive index contrast integrated waveguides, which are characterized by refractive index contrast values higher than 1%, up to approximately 40%, 5 depending on the specific application.
• These integrated waveguides allow fabricating very compact devices, because waveguide patterns with small bending radii, down to few microns, can be formed without incurring in high losses.
• high refractive index contrast integrated waveguides are made of semiconductor materials, such as, for example, InGaAsP/InP and AlGaAs/GaAs .
• Semiconductor waveguides feature refractive index differences larger than lxlO "2 (by comparison, in glass optical fibers the
• the waveguides must have rather small cross sections, which implies small optical field dimensions.
• the dimensions ratio between the mode in the waveguide and that in a fiber coupled thereto can be very low, and the overlap integral between the modes supported
• planar integrated waveguides with index contrast ranging from relatively low to very high values can be achieved, meeting the growing demand for high integration density
• 3D Si0 2 waveguide having an index contrast equal to 0.24 (in percentage approximately 16%) , consisting of a laterally tapered SiON waveguide having a step-wise decrease in thickness towards the taper point, which may have up to 0.5 ⁇ m residual width. 5 Also in this case, the vertical and lateral tapering makes the manufacturing process complicated. In order to simplify the manufacturing process, planar spot-size converter structures are required. Planar spot-size converters using periodic, quasi-
• Simpler planar spot-size converters consist in a waveguide that is only laterally tapered, having a 5 lateral width that varies along a transition section thereof, possibly according to an optimized profile, towards an optimized value at an interface facet with an optical fiber, so as to maximize the overlap integral in the fiber-to-waveguide coupling.
• simpler planar spot-size converters consist in a waveguide that is only laterally tapered, having a 5 lateral width that varies along a transition section thereof, possibly according to an optimized profile, towards an optimized value at an interface facet with an optical fiber, so as to maximize the overlap integral in the fiber-to-waveguide coupling.
• simpler planar spot-size converters consist in a waveguide that is only laterally tapered, having a 5 lateral width that varies along a transition section thereof, possibly according to an optimized profile, towards an optimized value at an interface facet with an optical fiber, so as to maximize the overlap integral in the fiber-to-waveguide coupling.
• simpler planar spot-size converters consist in
• ID laterally tapered waveguide mode converters are based on the fact that, when the waveguide width is decreased below a given value, the width of the mode supported by the waveguide increases; thus, narrowing the waveguide towards the interface facet until a waveguide mode
• IS dimension comparable to the fiber mode dimension is attained allows achieving a high fiber-to-waveguide coupling efficiency, while preserving single-mode operation. It has been shown that these structures can enable
• IS lower optical rib waveguides including a substrate, a lower cladding coated over the substrate, a lower rib waveguide, a core, an upper rib waveguide and an upper cladding.
• the lower rib waveguide defines a stepped pattern existing partially only in a coupling
• 3D integrated waveguide coupling structures for example, the International patent application No. WO 02/42808 A2 describes the use of a tapered waveguide for forming an optical waveguide multimode-to-single mode transformer, for interfacing a laser, having a multi-mode output, to a S single-mode optical fiber.
• the mode transformer has a high refractive index core layer, e . g. made of SiON, surrounded by a lower refractive index cladding.
• the core layer includes a wide input waveguide section to accept a multimode, including a fundamental mode, light input.
• ID input waveguide section is coupled to a narrow output waveguide section by a tapered region having taper length enabling adiabatic transfer of the fundamental mode of the multimode light from the wide input waveguide section to the output waveguide section while suppressing
• the narrow output waveguide section supports a single mode light output comprising the fundamental mode.
• the input waveguide section and the tapered region comprises a ridge waveguide, having a ridge on the core layer, with a width of the ridge
• the integrated waveguide characteristics have to fulfill several requirements: for example, the waveguide geometric dimensions typically have to guarantee a monomodal operation, the refractive index contrast has to
• 3D be chosen so as to minimize radiation losses in bends and allow high integration density, the material birefringence must be compensated by means of form birefringence or in other ways.
• suitable S solutions to the problem of fiber-to-waveguide coupling losses need to be devised, so as to keep the losses at an acceptably low level . All these requirements are to be fulfilled with an eye at the fabrication process.
• ID are either too complicated to be manufactured, or the design thereof is difficult to be optimized, or, in the case of the simple laterally tapered buried waveguides, only the fiber-to-chip coupling efficiency is optimized (i.e., attention is mainly paid to the width of the tip
• ridge or rib integrated waveguides are to be preferred over other integrated waveguide structures, such as buried waveguides, because they offer to the designer of integrated optical devices
• 3D a higher flexibility.
• rib waveguides it is easier to design integrated waveguides that are optimized both from the viewpoint of the coupling efficiency with an external optical field, for example for coupling the integrated optical device with optical fibers, and from S the viewpoint of the other waveguide circuital requirements .
• the Applicant has realized that, when dealing with medium to high refractive index contrast structures, i.e., structures with refractive
• ID index contrast values ranging from approximately 1% to approximately 40% and, preferably, from approximately 1% to approximately 20%, rib waveguides are preferable to other integrated waveguide structures for the reason that the presence of the slab offers a further degree of
• the slab height can be exploited to enable the material birefringence compensation in favor of polarization-insensitive operation.
• a thick slab height can be exploited to enable the material birefringence compensation in favor of polarization-insensitive operation.
• ⁇ D slab allows high coupling coefficients and wider gaps in directional couplers in favor of higher tolerance to the technological process; on the other hand, an excessive slab height causes high radiation losses in small radii bends of the waveguides; the slab thickness also
• ⁇ S influences the coupling efficiency with optical fibers and the single mode operation.
• the different physical and geometrical parameters can be used to meet a set of different requirements: integration density,
• the integrated waveguide structure comprises a waveguide core for guiding an optical field, the waveguide core being formed on a lower cladding layer; the waveguide core comprises a waveguide core layer substantially coextensive to the lower cladding
• a waveguide core rib of substantially uniform height, protruding from a surface of the waveguide core layer opposite to a surface thereof facing the lower cladding layer, a layout of the waveguide core rib defining a path
• substantially coextensive means that the waveguide core layer has a surface extension sufficiently wide so that the optical field in the waveguide core layer is
• the waveguide core layer has a size of at least two times the maximum width at 1/e of the local optical field.
• the surface extension of the waveguide core layer is such as
• the integrated optical waveguide structure comprises a circuit waveguide portion in which the waveguide core
• 3D layer has a first width, adapted to guiding the optical field through an optical circuit, and at least one coupling waveguide portion adapted to coupling the circuit waveguide portion to an external optical field.
• the coupling portion comprises a terminal waveguide S core rib portion having a second width lower than the first width and terminating in a facet, and a transition waveguide core rib portion optically joining to each other the waveguide core rib of the circuit waveguide portion and the terminal waveguide core rib portion.
• ID transition waveguide core rib portion is laterally- tapered so that a width thereof decreases from the first width to the second width.
• IS between the height of the waveguide core layer and an overall height of the waveguide core are chosen in such a way as to keep coupling losses arising when the external optical field is coupled to the integrated waveguide below a prescribed level .
• At least one among a value of the first width, a value of the overall height of the waveguide core and a value of the height of the waveguide core layer is chosen in such a way as to comply with requirements on the circuit waveguide portion depending
• At least one among a value of the second width and a value of the height of the waveguide core layer is instead chosen in such a way as to achieve a prescribed efficiency in the coupling of the integrated waveguide to an external optical field having first field
• the circuit waveguide portion may be designed to support an optical field of second field dimensions equal to or lower than the first field dimensions; the coupling waveguide portion performs a field dimensions adaptation S for adapting the second field dimensions to the first field dimensions .
• the circuit waveguide portion is designed in such a way as to support a single-mode optical field. However, this is not
• the integrated circuit waveguide is designed in such a way that a ratio of the first field
• IS dimensions to the second field dimensions falls in the range from approximately 1 to approximately 3.
• the lower cladding layer has a first refractive index
• the waveguide core has a second refractive index
• an upper cladding covering the waveguide core has a
• the first, second and third refractive indexes are such that a refractive index contrast between the waveguide core and the lower and upper claddings falls in the range from approximately 1% to approximately 20%,
• the waveguide core is made of silicon oxynitride (SiON) ; the lower cladding layer is made of silicon dioxide; the upper cladding may be made
• a length of the transition waveguide core rib portion is chosen in dependence of a ratio between the first width and the second width.
• S chosen to be at least equal to a minimum length that, expressed in microns, is given by the formula (l- W/W 0 ) *500.
• the terminal waveguide core rib portion preferably has a length chosen to be the shortest possible length
• the length of the terminal waveguide core rib portion may be determined on the basis
• the length of the terminal waveguide core rib portion is chosen to be approximately equal to a value that, expressed in microns, is given by the formula L teC exp(-
• ⁇ S integrated optical waveguide of a type comprising a waveguide core for guiding an optical field, formed on a lower cladding layer, wherein the waveguide core comprises a waveguide core layer substantially coextensive to the lower cladding layer and having a
• the coupling method comprises providing at least one coupling waveguide portion, designed for coupling an external optical field to a circuit waveguide portion in which the waveguide core rib has a first width.
• the coupling waveguide portion comprises a terminal waveguide core rib portion having a second width lower than the first width and terminating in a facet, and a transition waveguide core rib portion optically joining to each other the waveguide core rib in the circuit
• the transition waveguide core rib portion being laterally-tapered so that a respective width decreases from the first width to the second width.
• the second width and the first width, and a ratio between the height of the waveguide core layer and an overall height of the waveguide core are chosen in such a way as to keep coupling losses arising when the external optical field is coupled to the integrated waveguide below a
• At least one among a value of the first width, a value of the overall height of the waveguide core and a value of the height of the waveguide core layer may be chosen in such a way as to comply with
• At least one among a value of the second 'width and a value of the height of the waveguide core layer is chosen in such a way as to achieve a prescribed efficiency in the coupling of an S external optical field having first field dimensions to the integrated waveguide.
• a process for manufacturing an integrated optical waveguide structure comprising: ID forming a lower cladding layer over a substrate; forming a waveguide core on the lower cladding layer, wherein said forming the waveguide core comprises: forming a waveguide core layer substantially coextensive to the lower cladding layer and having IS substantially uniform thickness, and forming a waveguide core rib, protruding from a surface of the waveguide core layer opposite to a surface thereof facing the lower cladding layer, said waveguide core rib having a substantially uniform height, the ⁇ D waveguide core rib having a layout defining a path for the guided optical field.
• Said forming the waveguide core rib further comprises : forming at least one coupling waveguide portion
• ⁇ S designed for coupling an external optical field to a circuit waveguide portion in which the waveguide core rib has a first width.
• Said forming the at least one coupling waveguide portion comprises in turn: forming a terminal waveguide core rib portion having 3D a second width lower than the first width and terminating lb in a facet, and forming a transition waveguide core rib portion optically joining to each other the waveguide core rib in the circuit waveguide portion and the terminal waveguide S core rib portion, said transition waveguide core rib portion being laterally-tapered so that a respective width decreases from the first width to the second width.
• said forming the waveguide core comprises : ID forming a material layer over the lower cladding layer, and selectively removing the material layer to define the waveguide core layer and the waveguide core rib. Expediently, the terminal portion and the transition IS portion are formed simultaneously with said forming of the waveguide core rib .
• Figure 1 is a schematic illustration of a planar integrated optical waveguide according to an embodiment of the present invention
• ⁇ S Figure 2 is a diagram showing the variation of the coupling efficiency between two circular gaussian optical fields (in ordinate) , one in an optical fiber and the other in an integrated optical waveguide, as a function of the ratio of the field diameters at 1/e (in abscissa, 30 logarithmic scale)
• Figure 3 is a diagram showing the width and height at 1/e (in ordinate) of an optical mode supported by the waveguide of Figure 1, as a function of a width of the waveguide (in abscissa)
• S Figures 4A, 4B and 4C show contour plots of waveguide-to-fiber coupling losses simulated for the waveguide of Figure 1 as a function of the ratio of a waveguide core layer
• IS Figure 5A is a diagram similar to those of Figures 4A, 4B and 4C, showing contour plots of average coupling losses calculated from the coupling losses values depicted in the diagrams of Figures 4A, 4B and 4C;
• Figure 5B is a diagram similar to those of Figures
• FIGS. 7A and 7B are diagrams showing the measured lfl coupling efficiencies (in ordinate, dB scale) as a function of the fiber-to-waveguide misalignment along the horizontal axis and, respectively, the vertical axis (in abscissa, ⁇ m) ; and S Figure 8 schematically depicts an exemplary integrated optical device, in which a waveguide structure according to an embodiment of the present invention is exploited. Throughout the different drawings, identical
• FIG. 1 a planar integrated optical waveguide structure according to an embodiment of the present invention is schematically shown. More precisely, only a small portion of a waveguide 101 is depicted in the drawing, namely a waveguide portion proximate to an
• ⁇ D edge or tip 103 of the waveguide 101 intended to be coupled to, e . g. , an optical fiber 105 (more generally, to an external optical field, either guided or not) .
• the waveguide 101 is integrated in a chip 107 in which one or more optical components (not shown in Figure
• 3D 1.45 is formed by Chemical Vapor Deposition (CVD) , ⁇ particularly Plasma-Enhanced CVD (PECVD) , and has a thickness of some microns.
• CVD Chemical Vapor Deposition
• PECVD Plasma-Enhanced CVD
• a waveguide core 113 is formed, having a refractive index n core .
• the S waveguide core 113 made for example of silicon oxynitride (SiON) , having a refractive index n core that falls in the range from approximately 1.45 to approximately 2, is formed by depositing a SiON layer on the lower cladding layer 111, e . g. by CVD and,
• the deposited SiON layer is patterned, so as to form a core base layer (in jargon, a slab) 113a, of substantially uniform height t
• a core ridge or rib 113b of height (h-t) , where h denotes the overall height of the waveguide core 113.
• a birefringence compensating layer (not shown in the drawing) can be
• the birefringence compensating layer may be made of silicon nitride (Si 3 N 4 ) , formed by Low-Pressure CVD (LPCVD) .
• An upper cladding 115 of refractive index n uc covers
• the upper cladding 115 can be a material layer, for example made of Si0 2 , similarly to the lower cladding layer 111 (in which case the upper cladding refractive index n uc and the lower cladding refractive index n ⁇ c coincide) .
• the upper cladding refractive index n uc and the lower cladding refractive index n ⁇ c coincide can be a material layer, for example made of Si0 2 , similarly to the lower cladding layer 111 (in which case the upper cladding refractive index n uc and the lower cladding refractive index n ⁇ c coincide) .
• the upper cladding refractive index n uc and the lower cladding refractive index n ⁇ c coincide
• 3D cladding 115 can be made of, e . g. , air (refractive index
• an optical field 121 propagates through the waveguide 101 being guided by and being substantially confined within the S waveguide core 113.
• the waveguide core rib 113b confines the optical field 121 upperly and laterally, and the layout pattern thereof determines the optical field path in a plane parallel to that of the core base layer 113a.
• the waveguide core rib 113b has a substantially uniform height (h-t) throughout the die.
• the waveguide core rib 113b has instead a variable width in different regions of the chip 107.
• the waveguide core rib 113b has a circuit waveguide core rib portion
• the circuit waveguide core rib portion 117a has a first width (circuit waveguide width) W 0 . Proximate to the waveguide tip 103,
• ⁇ D a laterally-tapered, transition waveguide core rib portion 117b, of length L and variable width, joins the circuit waveguide portion 117a to a tip waveguide core rib portion 117c, of length Li P and having a second width (tip waveguide width) W lower than the circuit waveguide
• the tip waveguide core rib portion 117c terminates in a facet 119 (typically, but not limitatively, a facet coincident with the chip perimetral boundary; more generally, an interface facet between a region of the space in which
• the layer 113 is present, and an adjacent region of space
• the layer 113 is absent, for example in correspondence of a groove formed in an area of the chip) , through which the waveguide 101 can be interfaced to an external optical field, e . g. carried by the optical S fiber 105, or can emit optical radiation.
• an external optical field e . g. carried by the optical S fiber 105
• the reduction in width of the waveguide core rib 113b in proximity of the waveguide tip 103 creates a mode spot-size converting structure, that widens the optical mode supported by the waveguide to dimensions comparable
• the waveguide core rib can have a larger width; by way of example, in an embodiment of the
• the width in the circuit waveguide core rib portion 117a can be the maximum width that still guarantees the single-mode operating condition.
• the circuit waveguide portion has a strong guiding action, at least
• the profile and the length L of the laterally- tapered transition waveguide core rib portion 117b are
• ⁇ S chosen to avoid abrupt transitions between the narrower tip waveguide core rib portion 117c and the wider circuit waveguide core rib portion 117a.
• the length L and the profile of the laterally-tapered transition waveguide core rib portion are chosen to avoid abrupt transitions between the narrower tip waveguide core rib portion 117c and the wider circuit waveguide core rib portion 117a.
• 3D 117b may be determined according to any known design
• ID designer a great flexibility in the task of designing an integrated waveguide that satisfies the requirements in terms of both circuit waveguide characteristics and coupling efficiency with, e . g. , an optical fiber.
• an optical fiber e. g., an optical fiber.
• the height t of the slab 113a and the width W 0 of the circuit waveguide core rib portion 117a can be chosen in such a way as to satisfy circuit requirements for the waveguide, i.e., requirements deriving from the interaction of the waveguide with the
• the designer is left free to determine at least one among the height t of the slab 113a and the tip waveguide width W in such a way as to optimize the coupling efficiency between the waveguide and a selected optical fiber, having a given
• ⁇ S mean mode diameter ⁇ S mean mode diameter.
• a rib waveguide structure i.e., a waveguide structure in which the waveguide core comprises a core base layer, or slab, 113a, of uniform thickness, and core rib 113b, offers the
• FIG 2 a diagram of the coupling efficiency ⁇ (in ordinate) as a function of the ratio S f /S wg (in ⁇ S abscissa, logarithmic scale) is shown.
• the integrated waveguide 101 has a refractive index contrast ⁇ defined as :
• the refractive index contrast ⁇ depends on the refractive indexes n cor e. n ⁇ c and n uc ; in the exemplary case that the lower cladding and the upper cladding are S made of Si0 2 , a SiON waveguide core of refractive index equal to 1.4645 corresponds to a refractive index contrast ⁇ of approximately 1%, while a SiON waveguide core of refractive index equal to 2 corresponds to a refractive index contrast ⁇ of approximately 40%.
• ⁇ S fiber with mean spot size S f of approximately 10 ⁇ m is coupled to a waveguide with index contrast ⁇ approximately equal to 2%, having an average mode size S wg of approximately 4.6 ⁇ m, the resulting fields
• ⁇ S dimensions ratio K is approximately equal to 2.17, and a coupling efficiency of about 58% is achieved; 'when a waveguide with an index contrast ⁇ of approximately 6% is considered, having an average mode size S wg of S approximately 2.8 ⁇ m, the resulting fields dimensions ratio K is approximately equal to 3.17, and the coupling efficiency falls to 27%. A drastic drop of the coupling efficiency to 18% results from a waveguide with an index contrast ⁇ of approximately 8%, having an average mode
• IS sensitivity This can be achieved by properly varying the values of the parameters L, W, L tip , h and t.
• the coupling efficiency between the modes in the optical fiber and in the waveguide can be maximized by properly choosing the values for the width W of the
• both the vertical dimension S v wg and the horizontal dimension S h wg of the field vary with the waveguide tip width W; in particular, by decreasing the width W, the field horizontal dimension S h wg increases accordingly, tending to infinity as the width W tends to
• ⁇ b increases up to a value substantially equal to the vertical dimension of the field in the ' slab 113a, and, if the waveguide is symmetrical, cannot be increased any further.
• the field vertical dimension S v wg tends to infinity when the width W tends to zero.
• 3D waveguide 101 including the transition portion 117b
• the designer need to use a slab height t such that, in combination with a given waveguide height h, the ratio t/h is different from the optimum value, the coupling S losses can still be kept below desired levels by choosing values of the parameters W, W 0 , t and h such that the ratios W/W 0 and t/h are within prescribed ranges, which depends on the refractive index contrast. For example, considering again the diagram of Figure 4B, as long as
• the geometrical parameters W, W 0 , t and h are chosen in a way such that 0.3 ⁇ W/W 0 ⁇ 0.44 and t/h ⁇ 0.1, the coupling losses remain below 0.28 dB .
• ⁇ S W, W o , t and h need to fall within prescribed ranges; this means that the designer is left free to choose the absolute value of the geometrical parameters W, W 0 , t and h of the waveguide according to other requirements, such as monomodality, minimum bending radius, directional
• ⁇ D within which the variations in the resulting coupling losses can be kept within a predetermined tolerance as the refractive index contrast vary; for example, such a tolerance can be as low as ⁇ 0.01 dB, so that the coupling losses are made substantially independent from
• 3D contrast ⁇ takes values significantly higher than 8%, for 3D example 20% or even more (theorically, these results can be obtained for any refractive index contrast ⁇ , provided that the value of K is suitable, as discussed below) .
• the length L of the transition waveguide core rib portion 117b is chosen greater than a minimum value Lmin defined as :
• IS Lmin (1-W/W 0 )L 0 , where L 0 is the minimum length of the transition waveguide core rib portion 117b that guarantees an adiabatic transition even in case that the width W of the tip waveguide core rib portion 117c is chosen to be equal
• ⁇ S transition portions shorter than 500 ⁇ m are capable of ensuring a good adiabatic transformation of the optical field from the wider circuit waveguide portion to the narrower tip waveguide portion. Adiabatic transitions are not prevented by the use of longer waveguide transition
• the Applicant has taken 500 ⁇ m as the lower limit L 0 of the length of the transition portion in the most critical case of a width W reduced to zero.
• the length L ⁇ of the tip waveguide core rib portion 117c is chosen to be of the order of the hundreds of microns, and the effective length of this waveguide core
• IS rib portion is determined by taking into account the technological tolerances in cutting the wafer into individual dies and in preparing the chip edge face .
• L t i P is chosen to be equal to or greater than 100 ⁇ m.
• L t ⁇ p should be as
• ⁇ S transition waveguide core rib portion 117b is sufficiently long and W/W 0 is near 1, there is no reason for having a long tip waveguide core rib portion 117c to protect the structure from technological tolerances; on the contrary, a suitable guard has to be provided when
• the input optical fiber isS coupled to the tip waveguide core rib portion 117c, which ensures monomodality thanks to the extremely small cross section thereof.
• This fact guarantees that only the fundamental mode is excited in the circuit waveguide circuit waveguide core rib portion 117a, i.e., in the0 circuit waveguide, irrespective of any possible misalignment between the fiber and the waveguide.
• This feature becomes extremely useful when the circuit waveguide is dimensioned to have a cross-sectional area close to, or even above the second guided mode cut-offS (case in which a two mode propagation is possible) , but only the fundamental mode excitation is desired.
• the laterally-tapered transition waveguide core rib portion 117b had a cubic profile, and the integrated waveguide has been coupled to a small-core optical fiber with average mode dimension at l/e (S f ) equal to 3.6 ⁇ m.
• IS The circuit waveguide average mode dimension at 1/e (S wg ) was determined to be equal to 2.6 ⁇ m; consequently, the value of K was 1.38. From the choice of the geometrical parameters made, the value of the ratio W/W 0 was 0.33, that of the ratio ⁇ D t/h was 0.22. Referring to the diagrams of Figure 5A and 5B, a coupling loss slightly higher than 0.5 dB is expected.
• ⁇ D optical component 821 is schematically shown comprising, integrated in a chip 807, a ring filter 823, particularly, but not at all limitatively, a filter for high bit rate applications operating at a wavelength equal to 1550 nm.
• the ring filter 823 comprises an
• a waveguide 801 is integrated in the chip 807.
• the waveguide 801 has the structure shown in Figure 1, and includes an input mode spot-size converter 825a, an
• the input and output mode spot- size converters 825a and 825b are respectively coupled to S an input and an output optical fiber 805a, 805b.
• IS coupler 829 for particularly high bit rates applications, in respect of the technological tolerances in opening extremely narrow gaps between two waveguides .
• the waveguide structure of Figure 1 can be thus employed in any integrated optical component to enable high fiber coupling efficiencies and, at the same time, meet other requirements that must be satisfied.
• the main advantages of the described waveguide structure are the capability of achieving a high coupling efficiency with an appropriate optical fiber, at the same time satisfying requirements on the waveguide characteristics different from the coupling
• 3D efficiency e . g. requirements imposed by the particular integrated optical device or devices to be formed and with which the waveguide has to interact (circuital requirements) , weak influence on the coupling efficiency by tolerances on geometrical and optical parameters, low S sensitivity to fiber-to-chip alignment, and selective fundamental mode excitation, even when multimode (in particular, two-mode) circuit waveguides are employed.
• the described waveguide structure is particularly adapted for integrated waveguides characterized by medium
• index contrast values are adapted to realize integrated optical devices for Wavelength Division Multiplexing (WDM) and Dense WDM (DWDM) communication systems. With such index contrast values, waveguides with
• ⁇ S bending radii lower than 300 ⁇ m, and can be realized only if the index contrast is at least equal to approximately 5%.
• the described waveguide structure can be expediently exploited also for higher refractive index contrast values, up to approximately 40%.
• 3fl index contrast values for which the described waveguide structure may be exploited depends on the ratio K between the dimension of the optical field supported by the waveguide and the dimension of the external optical field S to be coupled to the waveguide field: as long as this ratio is relatively low, and particularly within approximately 1 and 3 , any refractive index contrast value is suitable.
• the described waveguide structure is symmetrical,0 and can be exploited in correspondence of both optical inputs and optical outputs of integrated optical devices .
• the invention can be applied in general whenever an integrated waveguide has to be coupled to an external optical field, either guided or not, and, particularly, an external optical field such that the ratio K of the dimensions thereof to the dimensions ofS the field supported by the integrated waveguide is relatively low, and preferably falls within the range from approximately 1 to approximately 3.
• the waveguide structure according to the present invention is easy to fabricate.
• the mode spot size conversion structure can be realized at the same time the rib core 113b is defined, by means of the same photolithography; no additional manufacturing steps are required compared to the manufacturing on a rib waveguide, only a peculiar layout of the photolithographic mask. This is a great advantage with respect to two-dimensional tapering known in the art, which involve more complicated processes with more steps . Alternative fabrication methods are however possible.

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